1. Field of the Invention
The present invention relates to a method of tagging signals used for leakage detection and measurement in cable television networks as well as apparatus for the detection and/or measurement of leakage sources tagged with this method.
2. Description of the Related Art
Cable television (CATV) systems are commonly used for the transmission and distribution of television and data signals from a headend (a place where the signals are prepared for transmission out onto the cable plant) to end users (subscribers) and for the transmission of data signals (e.g. Internet or telephony) from subscribers to the headend. Coaxial cable distribution systems typically use the 5 MHz to 1 GHz spectrum for the transmission of bi-directional analog and digital services. This spectrum shares frequency allocations with conventional broadcasters and many other terrestrial radio communication systems, including trunked radio systems (TETRA or P.25 used, e.g., by police or fire departments) and the air traffic control communication system.
One problem facing CATV operators is signal leakage. Signal leakage (sometimes called egress) refers to the transmission of signals through the discontinuities in a coaxial cable distribution network. The signals escaping from CATV networks can undesirably interfere with terrestrial radio communication systems, causing possible equipment malfunction (e.g., in civil aviation, emergency services) or problems with signal reception (e.g., interfered terrestrial TV channels). Moreover, each discontinuity radiating the electromagnetic energy from a CATV network forms a receiving antenna through which energy from terrestrial transmitters and other radio frequency sources (so called ingress) can enter the network. Most of leakage sources are also ingress sources. Ingress deteriorates the quality of signals propagating in the CATV network, especially from subscribers to the headend (return path services). Digital transmissions, typical for modern CATV services, are less susceptible to ingress entering the network than analog transmissions. However, a failure in the digital transmission is catastrophic—a subscriber may enjoy the program without any sign of a problem and suddenly experience a complete loss of the service. These are the main reasons for the need of leakage detection and measurement in CATV networks.
Leakage control is vital for CATV network operators. According to local regulations, CATV network operators should detect all leakage sources, determine the localization of leakage points, measure their absolute magnitude, and eliminate the sources which radiate stronger than a specified leakage limit. Currently, many leakage detection devices exist, which typically consist of an antenna and a receiver that is tuned to a frequency (referred as a test frequency) in the CATV signal bandwidth. Such a device usually has a signal strength (the absolute value of the electric field intensity) measurement circuit. A typical method of leakage source detection requires a signal strength measurement on the test frequency. If the signal strength measurement circuit detects a relatively large amplitude signal at a particular location, a leak may be indicated in or near that localization. A technician may use a leakage detector to pinpoint the source of a leak. A corrective action may be taken to remove the leak source from the network.
A drawback of the described leakage detection method lies in its inability to distinguish CATV signals radiated by the system under test from other signals in the same frequency band. For example, the detected signal may be radiated from terrestrial transmitters, other radio frequency sources or CATV networks placed in the same area and belonging to other operators.
There are some methods of differentiating the CATV leakage signals from other signals. The methods described in U.S. Pat. Nos. 4,072,899 and 4,237,486 require an unused CATV channel frequency which is modulated with a unique tagging signal. If the leakage meter detects a relatively large signal, it tries to isolate the distinctive tagging signal. The requirement of the unused channel frequency undesirably consumes valuable CATV network bandwidth and is unacceptable to CATV network operators today.
U.S. Pat. No. 5,608,428 describes a method in which a low frequency tagging signal is modulated onto an active (in-use) video carrier. The method can sometimes cause undesirable signal distortion for analog TV signals. U.S. Pat. No. discloses that it is possible to use the method for digitally modulated signals (like 64-QAM used for digital television and data transmission). However, the power measurement of digitally modulated signal, required for establishing the absolute magnitude of the leakage, is more complicated than the power measurement of a sinusoidal wave. Moreover, the measurement bandwidth for leakage should be very narrow (maximum several tenths of kHz around the test frequency;) in order to reduce interference from other CATV channels or terrestrial radiating sources. Usually, the smaller the bandwidth, the lower the amount of interference. The CATV digital signal has a bandwidth of a few MHz (8 MHz in Europe and 6 MHz in the U.S.A.), so it is much more susceptible to interfering signals. The measurement of the leak magnitude in such a broad bandwidth can give very inaccurate results. Furthermore, this method modifies signals transmitted in a CATV network.
U.S. Pat. Nos. 6,118,975, 6,307,593, 6,600,515 and 6,804,826 describe four leakage tagging methods that insert a tagging signal into a television signal only at times at which control information is present (vertical or horizontal synchronization pulses or quiet lines). The major drawback of all four methods is the fact that they can be used for tagging of analog television channels only. They also modify some signals transmitted in the network.
Recently, a new problem has appeared. Various parts of the network may belong to or be operated by various operators. For example, the main distribution part of a network—from the headend to a particular building—belongs to one operator (O1), and the home wiring inside the building belongs to a building owner (O2). A leakage source can be localized in the building. The mobile leakage patrol organized by O1 can detect this source and coarsely (within a 100-200 m range) determine its localization. The precise localization must be found by a technician and requires time and human resources. Due to the financial reasons, the operator O1, who has carried out the mobile patrol, is not interested in finding the precise localization of the leak sources not belonging to his part of the network. The standard tagging methods, like these described in the U.S. Pat. Nos. 4,072,899, 4,237,486, 5,608,428, 6,118,975, 6,307,593, 6,600,515 and 6,804,826, do not allow the spatial distinguishing of the CATV network part responsible for a particular detected leak source. They can recognize whether the leak source comes from the network under test, but cannot specify a particular network part where the leak source is localized.
Accordingly, there is a need to develop a leakage tagging method that does not require an out-of-use channel, does not modify the signals transmitted in a CATV network, and can be used for digital television channels (DVB-C, DVB-T or similar systems) and channels used for digital data transmission in the DOCSIS and EuroDOCSIS (or similar) systems. The method should also allow an accurate measurement of the absolute leakage magnitude and the determination of the CATV network part (subnetwork) where a detected leakage source lies.